5129
The Stereochemistry of the Base Hydrolysis of trans- [Co( NH, ) ( "NH, )XI '+ Ions D. A. Buckingham, I. I. Olsen, and A. M. Sargeson Contribution f r o m the Research School of Chemistry, Australian National University, Canberra, Australia. Received April 25, 1967 Abstract: The base hydrolysis of tran~-[Co(NH3)4(~~NH3)Cl]~+, tran~-[Co(NH3)4(l~NH3)Br]~+, and rrans-[Co(NH3)4(15NH3)N03]z+gives the same product ratio, 5 0 x cis- and 50x tran~-[Co(NH3)~(~~NH3)0H~] + a t 25'. The ratio is independent of [OH-] and no intramolecular rearrangement of tran~-[Co(NH3)~(1~NH3)OH~+ occurs in the time taken to effect hydrolysis. The common result fits the conclusion that a common intermediate is involved, and it is discussed in relation to the isotope fractionation and anion competition effects which have been observed for the same reactions.
also to the pentaammine series. The starting point of vidence has appeared recently t o support t h e contention t h a t the base hydrolysis of [ C O ( N H ~ ) ~ X ] ~ our + work5 is the demonstration b y Halpern and as= C1, Br, I, NO3) does take place b y an S N ~ C B sociates6 t h a t the trans-NH3 g r o u p i n C O ( N H ~ ) ~ S Ois~ + extremely labile. T h i s permits the substitution of mechanism (eq 1-3) involving an intermediate of re15NH3stereospecifically trans and leads to the prepara[ C O ( N H ~ ) ~ X ]+ ~ +OH[Co(NHa)&"HX]+ Hz0 (1) tion of [ C O ( N H ~ ) ~ ( ~ ~ N H ~ions ) X ] ~and + t h u s t o the [Co(NHa)aNHzX]++[Co(NHa)4NHzI2+ X(2) possibility of an examination of t h e stereochemistry of base hydrolysis i n cobalt(II1) pentaammine complexes. [Co(NH3)4NHzl2++ Hz0 [Co(NHa)sOHI2+ (3)
E ions (X
=
+
[CO(NH~)~NH~]* Y+-
+ HzO
--j
+ +
[Co(NHa)sYI2+
+ OH-
(4)
duced coordination number1p2 (2). The results used to s u p p o r t this proposal have relied on competition between isotopic H 2 0(eq 3) or different species (eq 3 and 4) in a q u e o u s solution f o r t h e supposed intermediate [ C O ( N H ~ ) ~ N H ~ ] ~Probably +. the most conclusive of these experiments involved the competition between 180H2,I60H2,'*OH-, and 160H- f o r the coordination site, and it w a s shown' by using the large isotopic equilibrium constant f o r the equilibrium 160Hz+ '*OH- J _ ''OH, + ''OH-
Experimental Section tr~ns-[Co(NH8)~(0H)(SO3)]. [CO(NH~)~SO&SO~ was prepared as described previously.' It was dissolved in excess concentrated HC1 at -20°, and the complex chloride was precipitated immediately with ethanol. This product, recrystallized from water with HC1 and LiCl at -20°, gave principally rrans-[Co(NH&(H20)S03]C1. The aquo sulphito complex was reconverted to the pentaammine salt by crystallizing it from aqueous ammonia solution with ethanol. [CO(NH~)~SO~]CI was then treated with a 3 % aqueous solution of LiOH for 5 min, and truns-[Co(NHa)4(OH)SOa]crystallized out slowly as fine plates on the dropwise addition
of ethanol. trans-[C0(NHa)~(~~NH~)Cl](Cl04)~. Freshly prepared truns[CO(NH~)~(OH)SO~] ( 5 g) was added to a solution of l5NH4C1 that the complexes did n o t react preferentially with (4 g) (96 I5N) and LiOH (2.4 g) in water (50 ml). The complex l60H- as required b y a bimolecular nucleophilic disdissolved and then trans-[Co(NH&( 16NH3)S03]CIcommenced to crystallize. After 10 min all the complex was crystallized by adding placement mechanism. The fact that the isotopic fracethanol. The isolated compound was washed with ethanol and tionation factors were constant f o r the hydrolysis as reether, dried in air, and treated with a solution of concentrated HC1 quired f o r a common intermediate also favored this insaturated with LiCI. The solution was gently warmed at first and terpretation. then heated on the steam bath until all the complex was converted to trans-[Co(NH&(16NHa)C1]C12 (3.3 g). The trans-chloride was Following this study it w a s s h o w n 2 t h a t the base hycollected and treated with an equivalent amount of AgClO4 in drolysis of the above ions i n t h e presence of anions Y M). The AgCl precipitate was filtered off and (=N3-,0Ac-,NOn-, NCS-) gave s o m e [ C O ( N H ~ ) ~ Y ] ~dilute + HC104 the complex perchlorate precipitated by adding excess concentrated as well as [CO(NH~)~OH]~+. Moreover the a m o u n t HC104. Anal. Calcd for [Co(NH&( l6NH3)CI](ClO&: N, 18.72; was essentially constant f o r each anion with the series H, 3.98. Found: N, 18.98; H, 4.24 (6632 50.7 in MHC1O4). fr~ris-[Co(NH3)4(~~NH~)Br](ClO4)~. rr~ns-[Co(NHa)~(~~NH~~ of complexes, as required f o r the production of a comHz0](C104)3was suspended in a KBr-HBr solution and heated on mon intermediate. It should also be mentioned that a steam bath until all the aquo complex was changed to bromo neutral species such as NH3(as well as anions) compete bromide. This was collected and converted to the perchlorate in f o r the supposed intermediate. the same way as for the chloro complex (e550 54.5 in M HC104). The isotopic and chemical competition experiments truns-[Co(NH~)~( 15NHa)N0,](ClO 4)z. rrans-[Co(NH3)4( l6NH,)CI](C10& (1.5 g) was dissolved in dilute HClO4 (50 ml, 10-3 M ) and are consistent with the conclusion that the base hydrolypoured into a solution (50 ml) of Hg(NO& (1.0 M ) in concentrated ses do involve intermediates having properties indepenHN03. The nitrato complex crystallized as the nitrate which was dent of those of the leaving group. The extent of recollected and washed with dilute "01, ethanol, and ether. The niarrangement accompanying base hydrolysis has been trate was then treated with cold concentrated HC104to convert it to
used i n other series of complexes4 as evidence t h a t for t h e m as well a similar mechanism operates. It seemed desirable therefore to extend t h e isomerization studies (1) M. Green and H. Taube, Znorg. Chem., 2,948 (1963). (2) D. A. Buckingham, I. I. Olsen, and A. M. Sargeson, J . Am. Chem. Soc., 88, 5443 (1966). (3) I . I. Olsen, unpublished work. (4) R. B. Jordan and A. M. Sargeson, Znorg. Chem., 4, 1849 (1965).
the perchlorate and finally recrystallized from water by adding concentrated HC104. Anal. Calcd for [CO(NH~)~( l6NH3)NO3](5) Professor Henry Taube has had an interest in this problem as well and has made a number of suggestions which significantly improve the analysis of the results and for which we are grateful. (6) J. Halpern, R. A. Palmer, and L. M. Blakley, J . Am. Chem. Soc.,
88, 2877 (1966).
(7) A. Werner and H. Gruger, Z.Anorg. Chem., 16, 406 (1898).
Buckingham, Olsen, Sargeson 1 Base Hydrolysis of ~~~~S-[CO(NH,)~('~NH~)XI '+Ions
5130
J
Figure 1. The 60-Mc pmr spectra of trans-[Co(NH&( 15NH3)C1](C104)2 (a), trans-[Co(NH&( 15NH3)NOa](C104)~(b), and truns[ C O ( N H ~ 1SNH3)0H2](C104)a )~( (c) in &-dimethyl sulfoxide (reference sodium trimethylsilylpropanesulfonate 0.0 ppm).
Figure 2. The 60-Mc prnr spectra of tr~ns-[Co(NHr)a('~NH3)X]"+ complexes in dilute D2S04 (4.0 X 10-3 N ) : (a) chloro, (b) bromo, (c) nitrato, (d) aquo (arbitrary scale in ppm - HOD signal -4.9 PPm).
In the parent chloro complex the trans-15NH3 doublet signals occur at 3.55 and 2.35 ppm in &-DMSO, (C10&: N, 20.95; H , 3.72. Found: N, 21.00; H, 3.74 (ejOO 56.4 Figure la, which are equidistant from the remnant of in M HC104). The chloro, bromo, and nitrato complexes the trans-14NH3 signal at 2.95 ppm. One-half of the were found to be free of aquopentaamminecobalt(II1) ion by means doublet almost coincides with the cis- 14NH3absorption of the cation-exchange method. Bio-Rad Analytical Resin Dowex (H+) 50W-X2, 200-400 mesh, was used. Some [ C O ( N H ~ ) ~ O H ~ ] -at 3.65 ppm. It is also apparent that a trace of the ( N o & was found in the initial nitrato nitrate, but this was removed cis-15NH3 doublet is present at 4.25 and 3.05 ppm during the recrystallizations. (JIs" = 72 cps). These assignments can be made with ~~U~~-[C~(NH~)~(~~NH~)(OH~)](CIO~)~. tr~ns-[Co(NH3)4(~~NH3)confidence since it has been shown elsewheres that the Cl](C104)2 (0.4 g) was dissolved in 25 ml of 0.3 M Hg(C104X in trans-14NH3 and cis-14NH3 signals are separated by 0.05 M HC104. After 5 min the aquo complex which formed was precipitated by adding concentrated HClO4 and cooling -43 cps and that the signal at high field (trans) collapses ( € 4 9 0 47.3 in HzO). during deuteration of the complex much faster than that Base Hydrolysis. trans-[Co(N H S ) ~ ( ' ~ N 3)CI](CI H O&, transdue to the twelve equivalent protons at lower field. It is [ C O ( N H ~ ) ~ ( ~ ~ N H ~ ) B ~ ]and (C~O tr~ns-[Co(NH&('~NH3)N03]~)~, clear from the spectra that the initial formation of the (C1Oa)z were hydrolyzed at 25" by dissolving the complex (0.3 g) in water (10 ml) and adding NaOH (1.0 ml, 2 N). After 5 min the chloro complex from trans-[Co(NH,),( 15NH3)S03]C1 solutions were acidified; the aquo complex crystallized on adding takes place substantially with retention of the original concentrated HC104and was collected immediately. In two experiisotopic distribution. The trans-aquopentaammine ments with the chloro complex 10 ml of 2.0 N NaOH and 100 ml complex was prepared by the Hg2+-inducedaquation of of 0.02 N NaOH were added to the complex solutions, 0.3 g/lO ml the corresponding chloro complex, a reaction which is and 0.3 g/100 ml, respectively. Base Hydrolysis in the Presence of Azide Ions. trans-[Coknown to occur without rearrangement of the ammonia (NH3)4(1sNH3)C1](C104)2 (1 g) was dissolved in 15 ml of dilute groups.s It is also apparent from the spectra of the HCIOl M ) and added to a solution of NaOH (50 ml, 0.1 N ) nitrato and aquo complexes (Figure lb,c) that they have saturated with NaN3 at 25". After 5 min the solution was cooled the trans configuration and within the instrumental error in an ice bath, and excess LiCl was added to precipitate the formed azido complex (yield 0.2 8). The chloride was converted to the contain the same amount of the cis isomer as the parent perchlorate as described for the chloro complex. The cold filtrate chloro complex. was treated with cold concentrated HBr; the aquo bromide salt In the trans-aquo spectrum (Figure IC), two broad crystallized and was then converted to the perchlorate in the usual absorptions occur at low fields, -4.7 and 5.8 ppm. way. These signals arise from H 2 0in the solvent and HzO on The pmr spectra of the complexes were obtained using a PerkinElmer R10 60-Mc spectrometer, The samples were dissolved in the complex, respectively, and the broadening arises d6-dimethyl sulfoxide acidified with a trace of D2S04 (98 %) (confrom proton exchange between the two species catacentration -200 mg/ml) or in dilute D2.504 solutions (concentration lyzed by adding D+. In "neutral" &-DMSO these -100 mgiml). The reference used in &-DMSO was sodium signals were sharp, and that due to HzO in the solvent trimethy Isilylpropanesulfonate. occurred at higher field -3.7 ppm. Similar observations apply to the spectra of the comResults plex ions in the dilute DzSO4 solutions (Figure 2). The The pmr spectra of t r ~ n s - [ C o ( N H ~15NH3)C1])~( 15NH3 doublet is sharp with JIsSH= 72 cps, but the (C104)2,~ ~ ~ ~ ~ - [ C O ( N H ~ ) ~ ( ~ ~ Nand H ~trans) N O ~ ] 14NH3 ( C ~ Oprotons ~ ) ~ ,produce a very broad absorption band [ C O ( N H ~ 15NH3)Hz0](C104)3 )~( measured in &-dimethyl which obscures the traces of trans-14NH3 present. sulfoxide are given in Figure 1. Figure 2 shows the However, the traces of cis-15NH3can still be observed pmr spectra of the same complexes and also of transalso with a coupling constant of 72 cps. It is also ap[ C O ( N H ~ 15NH3)Br](C104)Z )~( measured in dilute D z S 0 4 parent from Figure 2b that the bromopentaammine solution (4 X 10-3 M ) . In all instances the 14NH3 complex has the trans configuration. groups give rise to a single broad absorption which is The pmr spectra in &-DMSO of the aquo complexes much sharper in the DMSO solutions than in the dilute derived from the base hydrolysis of the chloro, bromo, DzS04. The ljNH3 group, however, produces a sharp and nitrato complexes at 25" (Figure 3a-c) indicate that doublet with a constant coupling constant of 72 cps for substantial rearrangement has occurred. Two sets of the trans species in both solvents. The doublet arises doublets appear for the cis- and trans-ljNH3 groups of from the coupling of the three equivalent proton spins equal intensity within the experimental error. The cis with the single spin on the I5N nucleus. Integration of doublet coincides with the trace of impurity doublet the spectra showed a ratio of 4 : 1 for the broad singlet: (3.25 and 4.45 ppm) found in the spectrum of transdoublet. These absorptions were equated to the twelve cis 14Nprotons and three I5N protons trans for all four (8) D. A. Buckingham, I. I. Olsen, and A. M. Sargeson, Ausrruliun J . Chem., 20, 597 (1967). molecular ions. Journal of the American Chemical Society
89:20 / September 27, 1967
5131 I
Figure 4. The 6 0 - M ~ pmr spectra of [CO(NH~)~('~NH~)OHZIFigure 3. The 60-MC pmr spectra of [CO(NH~)~('~NH~)OHZ](C104)s(a, c) and [CO(NH~)~(~~NH~)N~I(C~O~)Z (b, d) obtained from (ClO4)a obtained from the base hydrolysis of the trans-chloro (a, d), the base hydrolysis of the trans-chloro complex in the presence of bromo (b, e), and nitrato (c, f ) complexes (a-c: DMSO solutions; NaNa. d-f: DzS04,4 X lo-").
tains very little cis isomer. The aquo product contains [ C O ( N H ~ )15NH3)OH2](C104)3 ~( (Figure IC.) Also the more trans than cis isomer in contrast to the earlier reabsorption at 2.8 ppm due to some trans-14NH3has insults, but the experimental conditions have also been creased substantially in the products. Both the I5NH3 altered markedly. doublets and the cis- and trans- 14NH3absorptions were integrated for a series of spectra from several experiDiscussion ments and showed that the amounts of cis- and transThe most significant result of this investigation is that [CO(NH,),('~NH,)OH~]~+ were equal (*5 %). the amount of cis- and trans-aquo products arising from Similar conclusions arise from the analysis of the the base hydrolysis is the same for the chloro, bromo, spectra of the aquo products in DzS04(4 X IO-, N> and nitrato complexes. This is required if a common (Figure 3d-f), although one branch of the cis doublet is intermediate is involved. The result is consistent with obscured by the HOD signal. An attempt to separate the common 018/016 fractionation factors and with the these signals by moving the HOD absorption downfield common competition ratios observed for anionic species by addition of more DzS04 led to precipitation of the and water molecules reacting with the supposed intercomplex in the nmr tube. mediate. exchange in [CO(NH~)~OH]*+ is Although the While the interpretation of the isotope fractionation known to be slow in dilute alkali,g the possibility of an experiments seems unequivocal, it has been suggested intramolecular rearrangement in basic solution has to be that the competition experiments might also be interconsidered. To check this possibility tran~-[Co(NH~)~preted by an S N ~ C Bmechanism where the entering ( 15NH3)OH2](C104)3 was dissolved in 0.2 N NaOH for anion is insensitive to the leaving group. lo The present the time required for base hydrolysis and then recovered, results would also require the stereochemistry of the The pmr spectra before and after this treatment were course of the S N ~ C B reaction to be independent of the identical, and, since we know the rate of water exchange leaving group. To rationalize the three sets of data in with [ C O ( N H ~ ) ~ O His~ ]also ~ + slow in acid solution,g terms of an S N ~ C mechanism B requires a set of multiple the stereochemical rearrangements observed here must coincidences which seems to us improbable. The more have taken place during the base hydrolysis reactions. obvious and satisfying explanation for the common reAlso the base hydrolysis was carried out at three [OH-] sults from the different sources is that the experiments concentrations, 1.0,0.2, and 0.01 M , without any change embody a common intermediate. in the concentration of the cis and trans products. There has been considerable speculation about the The competition of species other than water for the stereochemistry of the supposed intermediate in the base intermediate was also examined. trans-[Co(NH3),hydrolysis either as a square pyramid or a trigonal (15NH3)Cl](C104)2was hydrolyzed in the presence of bipyramid (Figure 5). The latter has been favored" -4 N NaN3 in 0.1 N NaOH, and both the azido and because of its ability to be stabilized through a p r - d r aquo products were recovered. The pmr spectra of bond formed between the lone electron pair on the these species are given in Figure 4 measured in both amido group and the vacated d orbital on the metal ion &-DMSO and dilute D2S04. The azido complex in the trigonal plane. For the square pyramid (Figure clearly has the frans configuration. One-half of the 4), the opportunity for the r stabilization does not arise frans-15NH3doublet is obscured by the cis-14NH3signal since the metal ion orbitals with the correct symmetry to in the d6-DMS0 solution but both signals appear for the (10) Referee for ref 2. D2S04solutions. It is also clear that this product con(9) H.R. Hunt and H. Taube, J. Am. Chem. SOC.,80,2642 (1958).
(11) R. G. Pearson and F. Basolo, J. Am. Chem. Soc., 78, 4878 (1 956).
Buckingham, Olsen, Sargeson / Base Hydrolysis of trans-[Co(NH3)4("NHa)a
2+
Ions
5132
NH3 ’c
Figure 5. Possible five-coordinated intermediates in the base hydrolysis of [Co(NH&XI2+ complex ions.
overlap with the p orbital on the amido group are filled. It would be expected that the square pyramid would lead largely to retention of the original isotopic distribution by attack of the incoming group at the vacated coordination site, although some attack of the nucleophilic reagent might occur at the “collar” of the d,$orbital above the square plane leading to some rearrangement. In the trigonal bipyramid, substantial rearrangement would be expected from attack at the three sides of the trigonal plane. The attack need not be statistically disposed since the bipyramid is not symmetrical unless the amido group rearranges rapidly compared with the coordination of HzO. Such a rearrangement of the amido group must be governed by the rate of proton release from the other ammonia groups coordinated if the amido group is stabilized only in the trigonal plane. It is difficult to assess this rate for the intermediate but the [ C O ( N H ~ ) ~ Xcomplexes ]~+ of the same charge have rates of proton exchange12 in the vicinity of 107 M-l sec-l. If this value is assumed to be not greatly different from that for the intermediate and the rate of reaction of the intermediate is largely diffusion controlled, -lolo sec-l, it would seem that the amido group would not have time to rearrange before the intermediate reacted. Some support for this possibility might be adduced from the fact that there is no change in stereochemistry for the hydrolysis in the range 0.01-1.0 M OH- which extends the rate constant differences quoted above from 1Oloto IO5 sec-l. It has been observeds*13furthermore that the transammonia group exchanges its protons faster than the cis groups in [ C O ( N H ~ ) ~ Xcomplexes, ]~+ and it seems likely that this rate difference also reflects the acidity of the trans over the cis NH3 groups. We suggest therefore that the amido group is formed trans to the leaving groups and that this arrangement is preserved for the short lifetime of the intermediate. If the argument relating to H-D exchange and the absence of amido group rearrangement is accepted, and if the -NH2 group is in the trigonal plane, then proton loss from an NH3 group which was cis to the original substituent is eliminated by the results. A trigonal bipyramid of the form (12) J. W. Palmer and F. Basolo, J. Phys. Chem., 64,778 (1960). (13) P. Clifton and L. Pratt, Proc. Chem. Soc., 339 (1963).
would give equal amounts of cis and trans products by the equivalent paths a and b, and the additional product by path c must add to the cis form. If the argument relating to H-D exchange and amido group rearrangement is not accepted, then the lSNatom is disposed about all five positions equivalently, and the statistical distribution of the entering group is 4:1, cis:trans. Clearly the results do not support this proposal either. The equal amounts of cis and trans products formed show that the addition of water in the trigonal plane is not statistical and that attack at the site trans to the isotope is slightly preferred. Attack by nucleophilic reagents at Co is less likely in the positions cis to the double bond than in the trans position, since the double bond efficiently concentrates electronic charge between the Co and amido N centers. The striking result that the azido product is largely trans while the concomitant aquo product contains both cis and trans isomers might be rationalized by assigning N3- a marked preference for attack trans to the amido group in contrast to the less selective attack by the H 2 0 molecule. However, this experiment alone does not appear to advance our knowledge of the structure of the intermediate, although the stereochemistry in general suggests a preference for the trigonal bipyramidal intermediate over the squarepyramidal form. One interesting result which is not related to base hydrolysis but which has some bearing on the mechanism of the induced aquation reactions is the formation from trans-[Coof ~~U~S-[CO(NH~)~(~~NH~)NO~]~+ (NH3),( l5NH3)C1I2+ Hg2+with retention of configuration. It has been contended p r e v i ~ u s l y ’ ~that ~ ’ ~these reactions give rise to a square-pyramidal intermediate [ C O ( N H ~ ) ~which ~ + ] then competes for both H 2 0 and other anions in solution. Also for a series of such induced aquations full retention of configuration is observeds for the entry of H20. It has now been shown that if NO3- also competes for the coordination site the same stereochemical course is observed for the formation of the nitrato ion, a result which is consistent with the previous evidence for the presence of the five-coordinate intermediate in these induced aquations. Acknowledgment. The authors thank the Microanalytical Unit of the John Curtin School of Medical Research for the N and H analyses and Mr. S. Brown for the pmr spectra.
+
(14) F. A. Posey and H. Taube, J . A m . Chem. Soc., 79,255 (1957). (15) D. A. Buckingham, I. I. Olsen, A. M. Sargeson, and H. Satrapa, Inorg. Chem., 6 , 1027 (1967).
Journal of the American Chemical Society J 89.20 J September 27, 1967
